1. Field of the Invention
The present invention relates to an image forming apparatus for forming images, such as an electrophotographic printer, a copying machine or the like.
2. Description of the Prior Art
A conventional electrophotographic image forming apparatus for forming images by using an electrophotographic process comprises a drum-shaped or belt-shaped electrophotographic photosensitive member 1 for forming images, a charging device 2 for uniformly charging the photosensitive member 1, an exposure device 10 for forming a latent image on the photosensitive member 1, a development device 3 for developing the latent image on the photosensitive member 1 to form a developed image, a transfer device 4 for transferring the developed image onto transfer paper 8, a fixing device 9 for fixing the image on the transfer paper 8, a cleaning device 5 for cleaning off the residual developer remaining on the photosensitive member 1 after transfer, as shown in FIG. 14.
Particularly, in a small electrophotographic apparatus, the photosensitive member 1, the charging device 2, the development device 3, the cleaning device 5, etc. are integrated to form a process cartridge, which can be exchangeably mounted on the electrophotographic apparatus body including the transfer device 4 and the fixing device 9.
In the development process for forming an image on the photosensitive member 1, a potential difference is made between an exposed portion on the photosensitive member 1 and the other portion to selectively transfer the charged developer 7 onto the photosensitive member 1 from the development device 3. In this process, the developer 7 adheres to the exposed portion, and no developer 7 adheres to the unexposed portion.
However, in fact, the developer 7 is attracted by the unexposed portion because the charged developer 7 has nonuniformity in polarity, and thus a problem referred to as xe2x80x9cfoggingxe2x80x9d occurs in an image in which a trace amount of developer 7 adheres to a white image portion. Various studies have been made to decrease the fogging.
On the other hand, a decrease in fogging causes a reduction in constant amount of the developer on the photosensitive member. In such a system with an extremely small amount of the developer, when white images or similar images with a low rate of printing are continuously formed, the frictional force between the photosensitive member and the rubber portion of the cleaning blade is increased, thereby causing problems of blade noise due to vibrations of the photosensitive member and the cleaning blade, and peeling of the cleaning blade.
Further, the transfer paper 8 onto which the developed image on the photosensitive member 1 is transferred contains various additives for bleaching, etc. Some of the additives adhere to the photosensitive member 1 as a result of the repetition of contact between the transfer paper 8 and the photosensitive member 1, and cause a poor quality image. This is caused by the fact that the surface of the photosensitive member 1 is covered with water due to additives having relatively high water absorptivity under high humidity, and thus the resistance of the photosensitive member 1 is decreased, thereby causing a disturbance in the charge distribution formed by the latent image on the photosensitive member and thus poor charging.
Since image forming apparatuses have been used in many places all over the world with development in the image forming apparatuses, the image forming apparatus have been increasingly used in a high-humidity environment where no image forming apparatus has been previously been used. Further, paper containing large amounts of components, which adversely affect charging have been increasingly used as the transfer paper 8, and the problem with respect to poor charging has been increased.
In order to prevent the occurrence of poor charging, it is effective to apply a large amount of developer 7 to the photosensitive member 1. In this case, the developer 7 functions as an abrasive for the additives, which adhere to the photosensitive member 1, and thus has the effect of suppressing adhesion of the additives.
Therefore, fogging has the effect of preventing a flow of an image due to the poor charging, and a small amount of developer 7, which causes no faulty image, is uniformly supplied to the photosensitive member 1 in order to prevent a flow of an image.
On the other hand, another method for preventing the flow of an image is employed in which an image is formed on the photosensitive member and the developer is adhered thereto between the process performed on the photosensitive member, for forming an image on the transfer paper 8, and the next image forming process, while the abrasion effect of the developer 7 due to fogging is not employed.
However, in the above conventional example, the small amount of developer supplied onto the photosensitive member has a nonuniform distribution, and the amount of the developer supplied changes with its placement according to the circulation state of the developer in the development container.
In addition, the flow of an image does not uniformly occur, and, particularly, it easily occurs in a portion such as the position of a feed roller where a large amount of paper dust is present.
The amount of fogging can generally be controlled to some extent by appropriately designing the process of the development unit, controlling the electric field applied to the developer, and the like. However, the function of this method is to uniformly control the amount of fogging over the whole image, and thus it is impossible to produce fogging only in a specified portion of an image.
For a poor image, which can be solved by constantly supplying a small amount of developer, therefore, when the amount of fogging is determined to the specified portion, even if fogging in the specified portion of an image is unrecognizable, fogging more occurs in the other portion, thereby causing the problem of finally forming a poor image.
When a constant amount of developer is supplied onto the photosensitive member between the processes for forming images on the transfer paper, there is the problem of consuming an excess of developer.
An object of the present invention is to solve the above problems and provide an image forming apparatus which can form a good image without a flow by supplying an appropriate amount of developer to a white image portion where no image is formed, while preventing improper friction of a cleaning blade.
In order to achieve the foregoing object, in accordance with a first aspect of the present invention, there is provided an image forming apparatus in which an electrostatic latent image is formed on an electrophotographic photosensitive member with a laser beam, and electrostatically developed with a developer to form a developed image, and the developed image is transferred to a transfer material to form an image on the transfer material, the apparatus comprising exposure means for normally increasing the reflection density only of a specified region in the horizontal scanning direction of the laser beam in a white image portion where no image is formed.
In accordance with a second aspect of the present invention, there is provided an image forming apparatus in which an electrostatic latent image is formed on the photosensitive member with a laser beam, and electrostatically developed with a developer to form a developed image, and the developed image is transferred onto a transfer material to form an image on the transfer material, the apparatus comprising exposure means for normally performing weak exposure only in a specified region of the photosensitive member in the horizontal scanning direction of the laser beam, regardless of exposure of a latent image by an exposure device for forming an image.
In accordance with a third aspect of the present invention, there is provided an image forming apparatus in which an electrostatic latent image is formed on the photosensitive member with a laser beam, and electrostatically developed with a developer to form a developed image, and the developed image is transferred onto a transfer material to form an image on the transfer material, the apparatus comprising exposure means for normally performing small spot exposure at intervals of predetermined amounts or more only in a specified region of the photosensitive member in the horizontal scanning direction of the laser beam, regardless of exposure of a latent image by an exposure device for forming an image.
In accordance with a fourth aspect of the present invention, the position of spot exposure by the exposure means in accordance with the third aspect of the present invention is never adjacent to the position of exposure for forming an image.
In accordance with a fifth aspect of the present invention, there is provided an image forming apparatus in which an electrostatic latent image is formed on the photosensitive member with a laser beam, and electrostatically developed with a developer to form a developed image, and the developed image is transferred onto a transfer material to form an image on the transfer material, the apparatus comprising exposure means for normally performing small spot exposure at intervals of predetermined amounts or more so that spots are never adjacent to the position of exposure for forming an image.
In accordance with a sixth aspect of the present invention, there is provided an image forming apparatus in which an electrostatic latent image is formed on the photosensitive member with a laser beam, and electrostatically developed with a developer to form a developed image, and the developed image is transferred onto a transfer material to form an image on the transfer material, the apparatus comprising a memory device for memorizing the integrated time of exposure by an exposure device for forming an image at each of positions in the horizontal scanning direction, and exposure means for forming images on the photosensitive member in spaces between respective transfer materials where no image is formed, according to the exposure time memorized in the memory means.